1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and an apparatus of detecting a signal based on a minimum mean square error (MMSE) in a multiple-input multiple-output (MIMO) system.
2. Related Art
A multiple-input multiple-output (MIMO) technology may be applied to a wireless communication system so as to satisfy fast data transmission that is a requirement of a next-generation mobile communication system. The MIMO technology increases channel capacity by using a multiple transmit antenna and a multiple receive antenna without using an additional frequency or transmission power. Further, the MIMO technology can be easily coupled with an orthogonal frequency division multiplexing (OFDM) technology strong against multi path fading. An example of technologies for implementing diversity in the MIMO system may include space frequency block code (SFBC), space time block code (STBC), cyclic delay diversity (CDD), frequency switched transmit diversity (FSTD), time switched transmit diversity (TSTD), precoding vector switching (PVS), spatial multiplexing (SM), or the like. A MIMO channel matrix according to the number of receive antennas and the number of transmit antennas may be decomposed into a plurality of independent channels. Each independent channel may be referred to as a layer or a stream. The number of layers is referred to as a rank.
The MIMO system can improve the channel capacity and the transmitting and receiving efficiency but has a problem of having a plurality of antennas mounted therein. The plurality of antennas may be easily mounted in a base station that can implement relatively complex hardware, but it is not easy to implement a plurality of radio frequency chains in a small user equipment in connection with a size and a cost. Therefore, most of the wireless communication systems to which the MIMO system is applied have two spatial streams.
A receive signal may be detected by calculating a minimum mean square error (MMSE) based log likelihood ratio in the MIMO system having the two spatial streams. A method of calculating MMSE based LLR according to the related art has a small amount of calculations, but when transmit probabilities of all the transmit symbols are the same, has performance lower than a method of calculating optimal LLR capable of achieving the optimal performance.
Therefore, a need exists for a method of calculating LLR based on new MMSE so as to improve reliability of MMSE based LLR calculation.